Hm, you should generally be able to hit Return to close a message box on Mac. Or, more specifically, to activate the default action in a dialog (the button whose background is the tint color of the application). Spacebar activates the button that has the focus ring around it, which may or not be the default button, but you can use Tab to move the focus around.
At least, this is the way it's always been on OS X and later. I'm not sure about classic Mac OS.
alt Hacker News
Hm, you should generally be able to hit Return to close a message box on Mac. Or, more specifically, to activate the default action in a dialog (the button whose background is the tint color of the application). Spacebar activates the button that has the focus ring around it, which may or not be the default button, but you can use Tab to move the focus around.
At least, this is the way it's always been on OS X and later. I'm not sure about classic Mac OS.